Nitrogen removal is a difficult problem facing municipalities and sewer authorities throughout the world. While there has been significant advancement in nitrogen removal through biological treatment, efficient and effective nitrogen removal in a cost effective manner remains challenging. This is especially true in view of low nitrogen limits now being promulgated by many municipalities and governmental bodies.
In its basic form, nitrogen removal is a two-step process that entails nitrification and denitrification. The nitrification process is carried out under aerobic conditions and involves the oxidation of nitrogen in the form of ammonia so as to form nitrate. The nitrification process is represented as follows:
            NH      3        +          CO      2        +          O      2        ⁢      →          bacteria      ⁢                            ⁢            NO      3        +          new      ⁢                          ⁢      bacteria      
Denitrification, on the other hand, is carried out under anoxic conditions and entails the decomposition of organic matter using nitrate ions as an oxidant. As a result of the decomposition, nitrate ions are reduced to free nitrogen which is given off in a gaseous form. This denitrification process can be represented as follows:
            NO      3        +          organic      ⁢                          ⁢      matter        ⁢      →    bacteria    ⁢                    N        2            ⁡              (        gas        )              +          new      ⁢                          ⁢      bacteria      
Membrane bioreactor (MBR) activated sludge systems have been used to remove nitrogen from wastewater. These systems will typically include one or more anoxic reactor, one or more aerobic reactor followed by an aerobic reactor having membranes therein that are submerged or immersed within the aerobic reactor. In the anoxic reactor, denitrifying organisms utilize available organic carbon in the wastewater to reduce nitrate-nitrogen (NO3) to nitrogen gas (N2). In the anoxic reactor, the desired electron acceptor is nitrate and the presence of other electron acceptors, such as oxygen (O2) will compromise the denitrification rate and the overall effectiveness of the denitrification process. In the aerobic bioreactor, influent ammonia (NH3), as discussed above, is converted to nitrate and the nitrate-rich mixed liquor is recycled to the anoxic reactor. Typically the anoxic reactor volume is 10% to 30% of the total bioreactor volume. In cases where submerged membranes are used, the membranes act as a solids separation unit, where permeate is drawn through the membrane and the excluded or separated solids are recycled along with the nitrate back to the anoxic zone. The immersed membranes are subject to fouling due to a biomass concentration gradient produced by the flux and the subsequent accumulation and dewatering mechanism acting on the solids. The area immediately under and adjacent to the membrane modules must be scoured continuously with compressed air in order to minimize the fouling. The high air scour flow rate required to effectively scour the membranes often results in a local high dissolved oxygen concentration, which is then recycled to the anoxic reactor along with the recycled biomass or sludge. This dissolved oxygen carryover effect results in a reduced denitrification rate due to the presence of the alternative electron receptor (O2) and a reduction of a readily available carbon source (soluble BOD). This results in the effluent having an elevated soluble nitrogen concentration. This effect becomes more evident as the treatment facility approaches design flow capacity and the actual hydraulic detention time in the anoxic reactor decreases. Since the volume, and detention time, of the anoxic reactor is small compared to that of the aerobic treatment reactors and zone, and recirculation rates are high (2 to 5 times influent flow rates), the high dissolved oxygen concentration in the recycled stream cannot be adequately reduced. In order to overcome this effect, an external organic carbon source must be added to the anoxic zone.